Investigation Methods on Existing Dam Embankments 5

[Mad Mike]

I've lately been doing a number of geotechnical investigations for the rehabilitation and raising of relatively small earth dams. Typically farm dams on minor streams, capacity usually less than 100 000 cubes. Invariably homogeneous dams constructed of clayey materials throughout. Part of these investigations involves determining the existing embankment composition, since drawings and details are rarely available for these old dams...

I've been paging through the USBR "Design of Small Dams" manual, and cannot find any specific guideline to good practice during the investigation stage. It's not impossible I've missed it somewhere.

These are all low budget investigations, and my standard practice is to excavate trial pits with a backhoe, but because I've never been taught what is good practice on dam embankments, I avoid excavating below the freeboard at crest level, and I also avoid excavating slots into the downstream toe. In these areas, I use a small-diameter hand auger and light dynamic probe tests. It's painfully slow...

I'd greatly appreciate any practical experience for those carrying out similar work on a regular basis. Perhaps there are published standards I've not yet uncovered. My budgetary constraints do not allow mechanical auger holes as an alternative testing means and similarly, there is never time to backfill the trial pits to a particularly high standard...these are just single day investigations out in the sticks somewhere, and I'd like to do them to the highest standard possible.

Cheers,
Mike

 

[BigH]

Mad Mike

I would suppose that this is the most recent and SOA available. In cores of dams, only auger and sonic drilling.

https://www.usbr.gov/tsc/techrefere...inesForDrillingInEmbankmentDams_April2014.pdf

 

[r13]

For rural small dams, I noticed that each state has their own inspection guidelines. An example of IL is here for your information. Link

 

[GeoPaveTraffic]

I would typically use hollow stem augers with a combination of split spoon samples and Shelby tubes. Then backfill with a cement-bentonite grout.

Only published guidance I know of is, no drilling fluids can be used.

Mike Lambert

 

[Mad Mike]

Thanks guys- every time I post on here, I'm glad that I did.

All the best,
Mike

 

[GeoEnvGuy]

You may also want to consider reviewing Geotechnical Engineering of Dams.

Looking at the bigger picture you are providing a dam raise or rehab design for small retaining structure. How much liability are you expected to take on if you are doing superficial samples, without understanding the foundation and the material right above the foundation along a critical failure surface. How do you know that the dam is filter compatible with the foundation and is not eroding away unnoticed.

The designs are simple with limited information or investigation the design will be more expensive to construct. If there is a decent understanding of the dam and foundation material with respect to failure behavior, gradation and shear strength than the construction will be cheaper.

My concerns for these types of project are the foundation materials of the raise/rehab, the lack of hydrology study to determine the maximum flood levels and flows as well as the quality of construction to ensure filter compatible gradations and compaction effort achieved.

 

[Mad Mike]

GeoEnvGuy- I appreciate these comments, and it's something I always have in the back of my mind. Your last point in particular is something I'm wary of.

Keeping in mind that these are low-volume reservoirs, and all situated on very small streams with no significant flood potential, I'm allowing myself to use the dam's historic performance as a gauge of their integrity- the three I'm currently looking at are all in excess of 20 years old, and in pretty good condition considering the upstream and downstream batters are considerably over-steep.

The rehabilitation generally involves increasing the crest width, flattening the batters both upstream and downstream, re-sizing of the spillway and a slight raising. The only foreseeable consequence of failure would be the Farmer's loss of a reservoir. I don't believe I carry any liability- I'm working beneath a dam Engineer who is designing the rehabilitation works- my scope is as the Engineering Geologist, to provide a ground report to assist his design. If I price in drilling and expensive testing, I will lose the Engineer as a client completely; 100% certainty...our industry is currently at the tail-end of the price-cutting regime and has been absolutely raped by Clients consistently awarding the cheapest bidder.

Granted, there is some risk involved for the Engineer, who would always seek to use the Geotech (me) as a scapegoat, but I'm also not sure you quite appreciate how 3rd world the environment is around here (I may be wrong)- the Farmers very often do the earthworks themselves instead of paying competent Contractor's...and my whole feeling is that, provided there is no major risk associated with the failure, these old hands-on guys should go ahead and do it...they know the risks! Whenever there are unknowns, I state them clearly in my reports- I never pretend to be all-knowledgeable and I think that's one of the most important points I've learnt over the years.

A few months back I dealt with a Farmer who had a fair-sized dam on a broad, crocodile and hippopotamus-infested river...his dam embankment had breached a year prior and he had reinstated the breach himself (15 000 cubes) without any professional advice whatsoever. He failed to bench the new earthworks in and his reinstatement then breached at the exact same location during the next major flood, so he called me out to inspect. The situation was very hairy, so I recommended he employ a dam Engineer to carry out the rehabilitation design and supervision...he scoffed at the idea- although he had plenty of money and the dam Engineer was good value, he insisted on doing it himself, even after his first attempt failed and cost him a fortune!!! We often get Clients like this, and I never lay down the 1st world standards on them. In his case, he would never even consider to sue me if his dam breached again...we still have good guys like this around.

PS- with all the pink stars in this thread, I'm guessing that Pelelo is following...

Cheers,
Mike

 

[cvg]

USBR Manual
Guidelines for Drilling and Sampling in Embankment Dams

https://usbr.gov/tsc/techreferences...inesForDrillingInEmbankmentDams_April2014.pdf